Experiments on the spread of rugose mosaic and leaf roll in potato crops in 1946

A series of experiments on the spread of potato rugose mosaic (virus Y ), and leaf roll, which has been in progress on a uniform plan since 1943, was ended in 1946. Mean values for thirteen centres in England and Wales showed that in 1946 69% of the infections with virus Y and 48 % of those with leaf-roll virus reached the tubers of Majestic potatoes by the beginning of August. There was usually little subsequent increase of rugose mosaic, but a late increase of leaf roll was associated with a relatively high initial spread. Three-quarters of the virus Y and over half the leaf-roll infections occurred within five plants distance of the source. There was no close correlation between the spread of either virus and the maximum number of Myzus persicae , either apterous forms on the plants or alate forms caught on adhesive traps, but centres with high trap catches in July and August showed pronounced late season spread of leaf roll. There were marked differences at different centres in the relative spread of the two viruses. The amount of spread and the gradients from source of infection of the two viruses are compared over the period 1943–6.     

THE CORRELATION OF APHID NUMBERS WITH THE SPREAD OF LEAF ROLL AND RUGOSE MOSAIC IN POTATO CROPS

An analysis of the results of experiments in different parts of England and Wales from 1941 to 1947 on the spread of potato leaf roll and rugose mosaic showed that leaf roll spread was correlated with the number of alate Myzus persicae (Sulzer) caught on sticky traps throughout the potato-growing season; there was some correlation with the maximum count of M. persicae per 100 leaves, but this possibly results from the correlation between trapped aphids and the number per 100 leaves. Spread of rugose mosaic (potato virus Y) was correlated to a lesser degree with number of M, persicae , perhaps because other aphid species are often vectors. With both diseases higher correlations were obtained when the infected plants were dispersed among the healthy crop than when they were placed together in a row. It is concluded that it is possible to predict the average health of potato stocks in the following year from average trap data; further work may enable the health of individual stocks to be predicted.     

ROGUING POTATO CROPS FOR VIRUS DISEASES

Removing virus-infected plants from plots of Majestic potatoes at Rothamsted on 2 July 1947 did not reduce the spread of leaf roll but reduced rugose mosaic (potato virus Y) to about one-fifth of that in plots rogued on 21 July or left unrogued. Roguing Arran Pilot potatoes on 16 June or 2 July reduced leaf roll to five-sixths of that in unrogued plots; roguing on 16 June reduced rugose mosaic to about half that in plots rogued on 2 July, and about a quarter of that in unrogued plots. Lifting Arran Pilot potatoes in mid-August reduced virus diseases to about two-thirds.
Roguing flattened the gradient (decrease in percentage plants diseased with increasing distance from the source of infection) with rugose mosaic, but had little effect with leaf roll. Evidently any plants prevented by roguing from contracting virus Y were near the initially infected plants.
In 1948, Majestic and King Edward potatoes at three places were rogued during 22–24 June and tubers were dug during 28–30 July and again at the end of the season. Leaf roll spread more in Majestic than in King Edward, and rugose mosaic spread more in King Edward. Roguing reduced the spread of both by about one-fifth at Rothamsted, but had no effect at Sutton Bonington. At Bretton, in the Derbyshire hills, roguing had no effect on leaf roll, but prevented the spread of rugose mosaic.
The small benefit occasionally achieved by roguing in the ware-growing districts of England does not make the practice economically worth while.     

THE INFLUENCE OF HEAT ON SOME APHIDS

The thermal death-points of five species of aphids removed from their host plants lay between 38 and 41°C., when tested for 1 hr. at 60% r.h . Many aphids alive after 1 hr. at high temperatures died within the next day; no Myzus persicae recovered and reproduced after 1 hr. above 37.5°C. Third and fourth instars and adult apterae withstood heat better than first and second instars and alatae. More aphids died at 90% r.h . than at 60% r.h , and more at 60% than at 30% r.h . Aphids kept at 15% r.h . for 4 hr. before being heated showed a higher mortality than those kept at 95% r.h . Aphids on plants withstood temperatures higher than their thermal death-point off the plant. Presumably aphids can continue to cool themselves by evaporation while feeding; also lower temperatures on the surface of transpiring plant tissues will aid survival.     

INFECTIVITY OF APHIDS BRED ON VIRUS-INFECTED CAULIFLOWER PLANTS

Caged cauliflower plants infected with either cabbage black ring spot virus (CBRSV) or cauliflower mosaic virus (CIMV) were colonized with Myzus persicae or Brevicoryne brassicae. Winged and wingless aphids that voluntarily flew or walked from these plants were transferred singly to healthy cauliflower or other brassica seedlings to compare their feeding behaviour and ability to transmit the viruses. Wingless aphids settled to probe more readily than winged, and B. brassicae was initially more restless than M. persicae. CIMV was more readily transmitted than CBRSV by both species, and B. brassicae rarely transmitted CBRSV. Wingless aphids transmitted less often than winged ones, and no wingless B. brassicae transmitted CBRSV, although they did CIMV. Fewer aphids transmitted CBRSV from old plants than from young ones, but plant age had little effect on CIMV transmission.     

THE DIFFERENT DISTRIBUTION OF TWO BRASSICA VIRUSES IN THE PLANT AND ITS INFLUENCE ON SPREAD IN THE FIELD

Previous knowledge provided no explanation for the greater prevalence of cauliflower mosaic than of cabbage black ring spot in field crops of cauliflower. Both viruses are spread principally by Myzus persicae and Brevicoryne brassicae , and both are transmitted equally readily from infected seedlings. Cabbage black ring spot virus has a much wider host range, and sap from infected leaves has a higher dilution end-point than sap from leaves infected with cauliflower mosaic virus.
At least part of the difference between the rate at which the two viruses spread in the field may be accounted for by the different manner in which they are distributed in old infected plants, and the effect this has on transmission by aphids. Cauliflower mosaic virus occurs in high concentration in all the new leaves produced by infected plants. Cabbage black ring spot virus, on the other hand, occurs mainly in the older leaves, and even there is localized in parts that show symptoms. Only in recently infected plants does cabbage black ring spot virus occur in young leaves.
After flying, most aphids alight on the upper parts of plants; they are therefore less likely to acquire cabbage black ring spot virus than cauliflower mosaic virus. It may be significant that cabbage, a host in which old leaves are in a more favourable position for alighting aphids than are those of cauliflower, is also often extensively infected with cabbage black ring spot virus.